1. Field of the Invention
This invention is related to a field effect transistor, especially related to a field effect transistor that can reduce short channel effect.
2. Description of the Prior Art
Generally, when the sizes of semiconductor devices are shrinking, the power consumption can be reduced and the response time can also be shortened relatively. Also, since the required material is reduced, extra manufacturing cost can also be saved. Therefore, how to shrink the sizes of semiconductor devices has always been an important topic when it comes to the development of semiconductor manufacturing. However, when the sizes of semiconductor devices are too small, for example, smaller than 90 nm, the short channel effect becomes more obvious. The drain induced barrier lowering, DIBL, caused by the short channel is one example of the short channel effect.
FIG. 1 shows a top view of the field effect transistor 100 according to a prior art. The field effect transistor 100 comprises the substrate 110, the drain 130, the source 140, the gate 150 and the channel 160. The substrate 110 includes the active region 120. The gate 150 is formed above the middle of the active region 120 and the channel 160 is formed directly under the gate 150. Because the channel 160 is placed directly under the gate 150, the boundaries of the channel 160 and the boundaries of the gate 150 are partly overlapping with each other as shown in FIG. 1. The channel 160 divides the active region 120 into two sections, the drain 130 and the source 140. Thus, the distance between the drain 130 and the source 140 is the length of the channel 160.
FIGS. 2 and 3 show the distributions of energy level among the source, the channel and the drain in the long channel field effect transistor 200 and the short channel field effect transistor 200′ respectively. The structure of the long channel field effect transistor 200 in FIG. 2 and the structure of the short channel field effect transistor 200′ in FIG. 3 are the same as the structure of the field effect transistor 100 in FIG. 1. The long channel field transistor 200 comprises the substrate 210, the drain 230, the source 240, the gate 250 and the channel 260 while the short channel field effect transistor 200′ comprises the substrate 210′, the drain 230′, the source 240′, the gate 250′ and the channel 260′. The difference between these two transistors 200 and 200′ is that the length of the channel 260 is longer than the length of the channel 260′ . In addition, in FIGS. 2 and 3, the dotted lines represent the energy level of the drains 230 and 230′ when there is no bias voltage applied and the solid line represent the energy gap of the drains 230 and 230′ when the same bias voltages Vd are applied. When there is no bias voltage applied on the drains 240 and 240′, the energy levels of the channels 260 and 260′ are higher than the energy levels of the sources 240 and 240′ and the energy levels of the drain 230 and 230′ for both the long channel field effect transistor 200 and the short channel field effect transistor 200′. Therefore, sufficient voltage must be provided to overcome the energy gap between the source 240 and the channel 260 and the energy gap between the source 240′ and the channel 260′ for transmitting the carriers from the sources 240 and 240′ to the channels 260 and 260′.
Although the energy level of the channel 260 can be partly lowered when the bias voltage Vd is applied on the drain 230, the energy level of the source 240 of the long channel field effect transistor 200 is not changed due to the longer length of the channel 260. That is, providing sufficient voltage is still necessary to overcome the energy gap between the source 240 and the channel 260 for transmitting the carriers from the source 240 to the channel 260. However, in the short channel field effect transistor 200′, not only the energy level of the channel 260′ is lowered, but also the energy gap between the source 240′ and the channel 260′ is lowered due to the short channel. The lowered energy gap makes it easier to transmit carriers into the channel 260′ for the short channel field effect transistor 200′, which also implies that, the leakage current is increased and the sub-threshold voltage can be changed with the bias voltage. In addition, it becomes harder to turn off the channel of the semiconductor device by the gate voltage when the sub-threshold swing increases.
Since the short channel effect can increase the leakage current and power consumption of the semiconductor devices and the sub-threshold swing can cause the difficulty of controlling the semiconductor devices, how to avoid the inconvenience caused by the short channel effect while shrinking the sizes of the semiconductor devices has become a critical issue to be solved.